Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Gross Derivative Assets and Liabilities by Type and Balance Sheet Location (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 6,135.6	$ 6,606.8
Derivative Liability, Fair Value, Gross Liability	6,010.1	6,054.5
Derivative, Fair Value, Net	54.1
Exchange-traded Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,443.6	3,777.7
Derivative Liability, Fair Value, Gross Liability	3,313.8	3,255.4
Over the Counter (OTC) Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,621.2	1,852.3
Derivative Liability, Fair Value, Gross Liability	1,650.7	1,842.9
Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	27.8	93.5
Derivative Liability, Fair Value, Gross Liability	20.6	90.2
Over the Counter (OTC) Foreign Exchange Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	892.2	808.0
Derivative Liability, Fair Value, Gross Liability	865.4	741.8
Exchange-traded interest rate contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	126.8	13.4
Derivative Liability, Fair Value, Gross Liability	136.0	10.2
Equity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	22.8	61.9
Derivative Liability, Fair Value, Gross Liability	21.0	114.0
TBA and forward settling securities [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1.2	0.0
Derivative Liability, Fair Value, Gross Liability	2.6	0.0
TBA securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.6
Derivative Liability, Fair Value, Gross Liability	(0.2)
Derivative Asset, Notional Amount	(194.6)
Derivative Liability, Notional Amount	(163.7)
TBA securities sold [Member] [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.4
Derivative Liability, Fair Value, Gross Liability	(2.0)
Derivative Asset, Notional Amount	(314.1)
Derivative Liability, Notional Amount	(563.8)
Forward settling securities purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0.1
Derivative Asset, Notional Amount	(163.4)
Forward settling securities sold [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(0.4)
Derivative Liability, Notional Amount	(286.3)
Deposits and receivables from exchange clearing organizations [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	76.2	(273.9)
Deposits and receivables from broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	(52.9)	(1.1)
Financial instruments owned [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	30.6	44.3
Payable to broker-dealers, clearing organizations and counterparties [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	1.6	0.0
Financial instrument sold, not yet purchased [Member]
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	54.1	84.4
Counterparty And Cash Collateral Netting Adjustment [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	(6,081.7)	(6,837.5)
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ (5,954.4)	$ (5,970.1)